* availability-distribution: Retry on fail on next block.

Retry failed fetches on next block when still pending availability.

* Update node/network/availability-distribution/src/requester/fetch_task/mod.rs

Co-authored-by: Andronik Ordian <write@reusable.software>

* Fix existing tests.

* Add test for trying all validators.

* Add test for testing retries.

Co-authored-by: Andronik Ordian <write@reusable.software>

* Remove stuff out of the runtime that does not belong there.

There might be more, but it is a start.

* White space fixes.

* Fix tests.

* Leave whitespace in ui tests alone.

* Add back zstd for no reason.

* Fix browser wasm (hopefully)

* Indentation fix.

* Prepare request-response for PoV fetching.

* Drop old PoV distribution.

* WIP: Fetch PoV directly from backing.

* Backing compiles.

* Runtime access and connection management for PoV distribution.

* Get rid of seemingly dead code.

* Implement PoV fetching.

Backing does not yet use it.

* Don't send `ConnectToValidators` for empty list.

* Even better - no need to check over and over again.

* PoV fetching implemented.

+ Typechecks
+ Should work

Missing:

- Guide
- Tests
- Do fallback fetching in case fetching from seconding validator fails.

* Check PoV hash upon reception.

* Implement retry of PoV fetching in backing.

* Avoid pointless validation spawning.

* Add jaeger span to pov requesting.

* Add back tracing.

* Review remarks.

* Whitespace.

* Whitespace again.

* Cleanup + fix tests.

* Log to log target in overseer.

* Fix more tests.

* Don't fail if group cannot be found.

* Simple test for PoV fetcher.

* Handle missing group membership better.

* Add test for retry functionality.

* Fix flaky test.

* Spaces again.

* Guide updates.

* Spaces.

* WIP: Whole subsystem test.

* New tests compile.

* Avoid needless runtime queries for no validator nodes.

* Make tx and rx publicly accessible in virtual overseer.

This simplifies mocking in some cases, as tx can be cloned, but rx can
not.

* Whole subsystem test working.

* Update node/network/availability-distribution/src/session_cache.rs

Co-authored-by: Andronik Ordian <write@reusable.software>

* Update node/network/availability-distribution/src/session_cache.rs

Co-authored-by: Andronik Ordian <write@reusable.software>

* Document better what `None` return value means.

* Get rid of BitVec dependency.

* Update Cargo.lock

* Hopefully fixed implementers guide build.

Co-authored-by: Andronik Ordian <write@reusable.software>